Typically for an alkaline battery is a gradually decreasing voltage when it discharges, which means that for some appliances, which need a high base voltage to operate, the appliance stops working even while the battery has still a bit of energy left.
For example, a flashlight might stop working with a battery because of the high need of voltage from its flashlight, while the battery does not have enough voltage left to power the flashlight it might still work for months in a remote control. If you would calculate the mAh of that battery only for the flashlight it is a lot less than if you would calculate it for the flashlight and remote together, which on his part would give a different mAh when using the battery only for the remote.
Which means to calculate the mAh of an alkaline you would need exactly to know for what appliance it is used and how much this appliance drains in current, and how much minimum voltage this appliance needs. This is different for every appliance.
If we would test the batteries in various appliances we would see that the outcome from exactly the same battery can vary 2.5 times in capacity. This means that without knowing exactly what the properties of a certain appliance are, the mAh can't be predicted.
This is very different from a rechargeable NI-MH battery, there the voltage is a lot more constant, which makes the discharge difference between appliances a lot smaller.